Diesel automobile has higher energy efficiency, and discharges less carbon monoxide and hydrocarbons, than gasoline one, and thus has been increased more and more since mid-1980s. Recently, however, it became the target of criticism because of its exhaust gas, particularly nitrogen oxides and particulate matters (PM). Accordingly, to solve the air pollution problem by the increase of exhausted PM amount from the increase of diesel automobile, each country applies a strict standard to PM exhaust. Thus, the automobile equipped with a filtering apparatus of exhaust gas has become an important issue in the market entry.
Since the late 1970's, diesel particulate filter (DPF) has been studied as an apparatus for filtering particulate matters in exhaust gas from diesel engine. However, before 1980, the studies were not active because the environmental regulation standard could be met by the development of engine design and the improvement of fuiel, even without equipping DPF. However, the regulation standard became stricter since the late 1980's, whereby the studies about DPF have been more active.
DPF should be able to trap particulates included in exhaust gas, and burn and totally eliminate the particulates before the pressure drop by the trapping affects the engine. Also, DPF should have durability and resistance to heat. Such DPF may be classified into three groups: honeycomb monolith filter, ceramic fiber filter, and metal filter. Among these, the honeycomb monolith filter is vulnerable to heat impact at high temperature, and so has a short life-span. The metal filter has such advantages that its production cost is low and its production is easy, but also has such disadvantages that its heat resistance and corrosion resistance are weak. Recently, therefore, the studies about fiber filters using ceramic fiber have been active. Such ceramic fiber filters are manufactured in the forms of foam, extruded article, and non-woven paper, but the foam and extruded article forms are vulnerable to heat impact, and the extruded honeycomb form has low gas permeability since it cannot have high porosity rate. The non-woven paper form is known to have high porosity rate, and so high efficiency for eliminating particulates.
To commercialize such non-woven filters comprising ceramic fiber, they should have very high mechanical strength enough to stand against the vibration of automobile, high porosity enough to lower the back pressure of exhaust gas, and uniform dispersion of regular-sized pores enough to raise the filtering efficiency of micro-particles.
When manufacturing conventional ceramic filters, the mechanical strength was increased by adding a large amount of inorganic binders to the slurry for preparing green paper. However, this process has disadvantages that the rolling-up and wave-shaping processes are difficult since the tensile strength and plasticity of the green paper are deteriorated, and the filtering performance is decreased since the porosity and mean pore size of the final ceramic filter are decreased.
WO 03/004438 A2 discloses a method for rigidifying a fiber-based paper substrate by impregnating fiber-based paper substrate with an impregnating dispersion containing colloidal nano-clay, followed by calcining, to increase the mechanical strength of ceramic filter, and optionally, further impregnating fiber-based paper substrate with alumina dispersion, zirconyl acetate solution, silica dispersion, etc., as a second impregnating dispersion, followed by calcining. However, this method has such problems that the nano-clay may be agglomerated, and the agglomerated inorganic particles such as clay or alumina may block some pores. Further, even the mechanical strength of the ceramic filter prepared by the above method is not enough to commercialize it.
U.S. Pat. No. 6,214,072 discloses a ceramic filter prepared by coating inorganic binder comprising colloidal silica, colloidal alumina, colloidal zirconia, or mixtures thereof. However, this method also has such disadvantages that the pores may be blocked by the inorganic particles, and the mechanical strength of the final ceramic filter is not satisfactory enough, as WO 03/004438 A2.
Also, U.S. Pat. No. 6,444,006 discloses a method for preventing the decrease of post-calcining strength by conducting heat treatment after uniformly depositing SiC precursors onto the surface of ceramic paper with using the Chemical Vapor Deposition. However, this method has such disadvantages that the process efficiency is very low because the Chemical Vapor Deposition should be used, and thus the production cost is very high.